(Not Applicable)
In modern coherent sensor systems such as radar and sonar, it is advantageous to represent the received signal as complex numbers. Complex signals are conventionally derived by separating real signals into in-phase (I) and quadrature (Q) components; a representation that is conducive to efficient digital signal processing. This technique is known as quadrature demodulation.
Typically, an intermediate frequency (IF) signal is split and mixed with coherent I and Q reference sources given by 2 cos xcfx89t and xe2x88x922 sin xcfx89t, respectively. After mixing, the products are low pass filtered to retain the difference frequency terms. As a result, the signal is split into I and Q terms which ideally combine to provide a complex output Y(t)=I(t)+jQ(t)=A(t)xc2x7ej"THgr"(t).
This ideal signal is obtained only if the gains of the I and Q. paths are equal and if the phase difference between channels is 90xc2x0. If the ideal signal is not attained, the resulting data is corrupted, ultimately producing ghosts and other undesirable artifacts in a radar image.
Practically, balancing of the I and Q channels is difficult to achieve and maintain, especially considering the wide bandwidths and high frequencies of modern high-performance radar systems. Existing techniques fall into one of two broad categories:
The first category attempts to measure the imbalance, and then correct the data accordingly. These techniques either have a limited capability to mitigate any but the simplest of phase and/or amplitude errors, or they impose a significant processing load on the system. (An imaging radar, such as a synthetic aperture radar (SAR), imposes a tremendous data processing requirement on a system even in the ideal situation. A requirement for additional processing of large SAR signals does not result in rapid image acquisition, and may be prohibitive in many applications.)
F. Churchill et al., xe2x80x9cThe Correction of I and Q Errors in a Coherent Processorxe2x80x9d, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-17-1, January 1981, pp. 131-137, discusses one of the first of such techniques. They used a test signal to determine the amount of correction to be applied to the channels. However, such a system is not dynamic in that the system can change as it operates, and this change would only be detected by degrading performance. Otherwise, to prevent this, periodic recalibration periods would need to be incorporated within the normal operation of such a system, resulting in missed data collection opportunities.
Another system using a test signal is discussed by N. Halwani et al., U.S. Pat. No. 5,315,620, which aims to avoid the need for high data sampling rates. The quadrature phase error is detected using a test signal and an error signal is generated. The received I and Q signals are applied to a correction network, along with the error signal, to minimize quadrature phase error between the I and Q signals.
The second category attempts to generate I and Q data using only a single analog to digital converter (ADC). These systems either require excessive video bandwidth (by a factor or two or more), or they suffer a need for excessive power (by a factor of two or more).
K. Ho et al., xe2x80x9cA Digital Quadrature Demodulation Systemxe2x80x9d, IEEE Transactions of Aerospace and Electronic Systems, Vol. 32, No. 4, October 1996, pp. 1218-1227, discloses an I-Q, demodulator based on low pass filtering of the input samples. They show that with a sampling frequency at least equal to 2B, the I components are the decimated input samples with appropriate sign changes and the Q samples are the low pass filter output of a frequency shifted input sequence. A finite impulse response low pass filter minimizes computation. A sampling scheme is also provided that downconverts without IF mixing.
The video-frequency offset technique involves using a single channel to process digital I and Q data in one channel. FIG. 1 shows a prior art system 2 where the received radar signal XIF(t)=cos(xcfx89IFt+"PHgr"(t)) is mixed in mixer 4 with a local oscillator signal of cos((xcfx89IFxe2x88x92xcfx80fs/2)t) down to a frequency fs/4, passed through a low pass filter 5 to limit the signal to IF bandwidth BIF, and sampled in analog to digital converter (ADC) 6 at rate fs, where fsxe2x89xa72BIF. The samples are then digitally mixed in mixer 7 to their final form with a digital local oscillator (DLO) signal of exp{xe2x88x92jnxcfx80/2}, which provides a repetitive input to mixer 7 of 1, xe2x88x92j, xe2x88x921,j.
One problem with this scheme is that it requires fsxe2x89xa72BIF, whereas separate IVQ channels and ADCs require only that fsxe2x89xa7BIF. (In a typical radar for which this circuit may be used, BIF may range from several MHz to several hundreds of MHz, where an additional factor of two for the ADC sampling frequency might be particularly problematic.) Consequently, this circuit requires more expensive ADCs and accompanying digitial circuits, which may not even be available, or must contend with the performance limits of a lesser IF bandwidth.
FIG. 2A shows the IF signal location in the 2-sided frequency domain at the input of filter 5 for the circuit of FIG. 1. As shown in FIG. 2B, at the output of filter 5, the real-valued signal is symmetric about DC. FIG. 2C shows the output signal after mixing with the DLO. This signal can be digitally filtered and decimated, if desired, to the equivalent of conventional I/Q channel data.
According to this invention, the spectrums of the desired balanced signals are separated from the error imbalance signals in a manner similar to the second category, but two ADCs are used to avoid the shortcomings denoted above.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a method of providing a balanced demodular output comprising providing an analog pulsed input; adding a variable phase shift as a function of time to the input signal, applying the phase shifted input signal to a demodulator; generating a baseband signal from the input signal and low-pass filtering the result; converting the baseband signal to a digital output signal; and removing the variable phase shift from the digital output signal to form a complex data output representative of the output of a balanced demodulator.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.